Scintillation cameras are widely used in the field of nuclear medicine for detecting lesions, cancerous growths, circulatory irregularities, and other abnormalities in the internal organs of a living subject. The basic scintillation camera is described in U.S. Pat. No. 3,011,057. In the operation of a scintillation camera, a patient is injected with a small quantity of a radioactive substance having an affinity for a particular organ or area of interest within the body of a living subject. The detector element of the scintillation camera, which includes a radiation transducer, is positioned adjacent to the area of the patient's body to be examined. Gamma rays produced by the radioactive disintegrations of the radioisotope administered pass from within the body of the patient to strike a planar scintillation crystal. In response to the impinging gamma rays, the scintillation crystal emits flashes of light. An array of photodetectors viewing the scintillation crystal responds to the light flashes for each detected radioactive event by generating electrical impulses. Each photodetector views an overlapping portion of the scintillation crystal. The strength of the electrical impulses from each photodetector may be directly related to the distance of the scintillation in the crystal from that photodetector. In this manner, the coordinates of interaction of gamma rays with the scintillation crystal in a two-dimensional coordinate system may be determined. By interposing a collimator between the scintillation crystal and the patient, the points of origin of the detected gamma rays within the body of the patient may be ascertained in the same two-dimensional coordinate system.
In conventional scintillation cameras, the detector construction involves the incorporation of a scintillation crystal subassembly. This subassembly includes a scintillation crystal of thallium-activated sodium iodide. The scintillation crystal is in the form of a disc with one face of the disc positioned in contact with a glass window, one-half inch in thickness formed of pyrex 7740 glass. This glass window is an optical window which permits scintillations to escape the sodium iodide and impinge upon the photodetectors. The remaining surfaces of the scintillation crystal are surrounded by an aluminum casing which is sealed to the glass window thereby entrapping the sodium-iodide crystal in a moisture free environment. This protection from moisture is necessary because sodium iodide is hygroscopic, and when moisture is absorbed by the sodium iodide crystal, the crystal becomes cloudy and unsuitable for use as a scintillation crystal in a scintillation camera.
The conventional use of the one-half inch thick glass plate represents a compromise among several requirements for a scintillation camera. The relatively large thickness of the glass window has heretofore been required to protect the sodium iodide crystal from mechanical stress during the assembly of the scintillation camera detector head. In the conventional manner of assembly, an optical coupling compound is coated either on the surface of the glass window or on a light conducting element sometimes referred to as a "light pipe". The scintillation detector subassembly and the light conducting element are then forced together with considerable mechanical pressure to obtain complete optical coupling between the light conducting element and the scintillation crystal. Later during the assembly process, and after the light conducting element has been coupled to the scintillation crystal subassembly, photodetectors are similarly positioned in optical contact with the light conducting element to form an array as previously described. Again, mechanical force is required to ensure a complete and uniform optical coupling between the photodetectors and the light conducting element.
In the conventional assembly of scintillation camera detectors, it has been found that if the glass window associated with the scintillation crystal subassembly is less than about one-half inch in thickness, flexing of the glass window occurs during optical coupling of a light conducting element and during coupling of the photodetectors to the extent that too much stress is placed on the sodium iodide crystal, and the crystal is likely to fracture. If this occurs, of course, the scintillation crystal is useless and must be replaced at considerable expense.
The relatively thick glass window used in conventional scintillation cameras has always been considered undesirable because it absorbs a significant portion of the light transmitted from the scintillation crystal. This light absorption is particularly severe for light traversing the glass window at angles other than at a path perpendicular to the glass window. The result is a degradation of resolution of the instrument. Alternatives have been sought to remedy the design shortcomings inherent in the relatively thick glass window of conventional commercial scintillation detectors. One alternative has been a one-half inch thick quartz window in place of the glass window. Quartz has a superior index of transmission of light from the sodium iodide crystal, but it also has a worse match of refractive index with sodium iodide than does the current glass window. This mismatch of refractive indices produces errors in the positional information transmitted by the photodetectors. Crown glass has also been considered as an alternative to the pyrex glass conventionally used. However, because of the need for mechanical rigidity, no major reduction in window thickness has been practical using this alternative.